Restoration failures resulting from tooth fracture are one of the primary obstacles to lifelong oral health. The overall hypothesis of our study is that the fracture of restored teeth is fostered by a reduction in the fatigue crack growth resistance of dentin with age. Our preliminary studies have shown that fatigue cracks are often present in the dentin of restored teeth and that the fatigue properties of dentin are a function of both the tubule orientation and patient age. Despite distinct changes in the structure of dentin with age, and identification that tooth fractures occur more frequently in seniors, the influence of aging on the fatigue properties of dentin has remained unknown. With the increase in partially and full dentate seniors, the role of aging on restored tooth failures has become increasingly important. We propose to quantify the fatigue crack growth properties of dentin as a function of patient age, gender and dentin tubule orientation, distinguish at what age the most detrimental changes take place, and identify, the fundamental mechanisms responsible for reduction in crack growth resistance. Fatigue crack initiation axnd growth will be evaluated in human dentin specimens prepared from the coronal dentin of extracted virgin molars. The mechanisms of crack initiation and cyclic crack extension will be characterized using a novel application of digital image correlation and evaluated in terms of the patient's age and gender. Changes in the structure and chemistry of dentin will also be evaluated using analytical microscopic techniques. Results of the investigation will provide a fundamental understanding of the fatigue crack growth properties of dentin that enable development of detailed structure-property relationships. Based on both quantitative results and an understanding of contributing mechanisms, we expect to identify the critical aspects of existing restorative treatments on tooth fracture and the potential need for new approaches in treatment of the aging dentate population. The results are expected to support development of clinical techniques for arresting cracks in tooth structure that involve both mechanical and chemical approaches to repair. Hopefully our findings will help identify approaches for strengthening the fatigue resistance of tooth tissues, regardless of age. These achievements should reduce the incidence of restorative failures associated with tooth fracture, resulting in billions of dollars in savings of health care costs, as well as a reduction in human suffering.